Digestion of lignocellulosic material with cyanide and bisulfite ionsat a ph below 7

ABSTRACT

A process for increasing the yield of pulp from lignocellulosic material which comprises treating lignocellulosic material with a solution containing cyanide and bisulfite ions. The process may be carried out in several embodiments, one of pulping the lignocellulosic material in the cyanide-bisulfite solution at conventional bisulfite pulping temperatures of about 170*C. Another embodiment is to treat the lignocellulosic material with the cyanide-bisulfite ion solution at a temperature below 130*C., preferably about 95*C., and follow by heating to conventional bisulfite digesting temperatures. Another embodiment of the process of this invention is to treat lignocellulosic material with a cyanide-bisulfite ion solution at a temperature below 130*C., withdraw the cyanide-bisulfite solution and introduce neutral or acid bisulfite digesting liquor and digest as usual. The process of treating lignocellulosic material with the cyanide-bisulfite containing solution is at a pH below 7 and is faster than by treating lignocellulosic material with a solution containing cyanide ions alone. The treatment of lignocellulosic material according to this process results in pulps having increased yield and improved brightness stability over conventional bisulfite cooks.

United States Patent [1 1 Falkehag et al.

[ 1 DIGESTION OF LIGNOCELLULOSIC MATERIAL WITH CYANIDE AND BISULFITE IONS AT A PH BELOW 7 [75] Inventors: Sten I. Falkehag, Mount Pleasant;

Alan M. Bills, Summerville; Harold L. Hintz, Charleston, all of SC.

[73] Assignee: Westvaco Corporation, New York,

[ Notice: The portion of the term of this patent subsequent to May 16, 1989, has been disclaimed.

[22] Filed: Jan. 10, 1972 211 Appl. No.: 216,811

Related US. Application Data [63] Continuation-impart of Ser. No. 33,044, April 29,

1970, Pat. No. 3,663,358.

1 *Oct. 2, 1973 Primary Examiner-S. Leon Bashore Assistant Examiner-Arthur L. Corbin Att0rneyErnest B. Lipscomb, Ill

[57] ABSTRACT A process for increasing the yield of pulp from lignocellulosic material which comprises treating lignocellulosic material with a solution containing cyanide and.

bisulfite ions. The process may be carried out in several embodiments, one of pulping the lignocellulosic material in the cyanide-bisulfite solution at conventional bisulfite pulping temperatures of about 170C. Another embodiment is to treat the lignocellulosic material with the cyanide-bisulfite ion solution at a temperature below 130C., preferably about 95C., and follow by heating to conventional bisulfite digesting temperatures. Another embodiment of the process of this invention is to treat lignocellulosic material with a cyanide-bisulfite ion solution at a temperature below 130C., withdraw the cyanide-bisulfite solution and introduce neutral or acid bisulfite digesting liquor and digest as usual. The process of treating lignocellulosic material with the cyanide-bisulfite containing solution is at a pH below 7 and is faster than by treating lignocellulosic material with a solution containing cyanide ions alone. The treatment of lignlocellulosic material according to this process results in pulps having increased yield and improved brightness stability over conventional bisulfite cooks.

6 Claims, No Drawings DIGESTION OF LIGNOCELLULOSIC MATERIAL WITH CYANIDE AND BISULFITE IONS AT A PH BELOW 7 CROSS-REFERENCE 'ro RELATED APPLICATIONS This application is a continuation-in-part of copending U. S. application Ser. No. 33,044 filed Apr. 29, 1970, now U.S.Pat. No. 3,663,358.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for increasing the yield of neutral and acid bisulfite pulps. More particulary, this invention is directed to a process for improving pulp yield of neutral or acid bisulfite pulp by reacting lignocellulosic material with a solution of cyanide ions and bisulfite ions.

2. Background of the Invention Lignocellulosic material (such as wood from coniferous and deciduous species of trees, the stalks ofbamboo and reeds, bagasse, and cotton fibers) principally contains lignin, cellulose, and hemicelluloses. The lignin and hemicelluloses are located both in the cell wall of the fiber and in the inter-fiber spaces, generally termed the middle lamellae. In chemical pulping prcesses utilizing lignocellulosic materials, lignin in the inter-fiber spaces must be dissolved, with as little damage as possible to the fibers, so thatthe fibers are readily separable. I-Iemicelluloses can be retained on the surface of the fiber, in which case they can influence the bonding properties of the fibers, and they are also retained in the cell wall of the fiber, improving the flexibility and thereby the papermaking properties of the pulp. It is consequently generally desirable to retain the hemicelluloses in order to produce paper products having superior strength characteristics.

In attempting to prevent degradation of cellulose during alkaline pulping it is known that the alkaline pulping liquor attacks the carbonyl end group of the cellulose chain causing peeling. This problem has been overcome by treating the cellulose with a cyanide containing solution as disclosed in U. S. Pat. No. 3,532,596.

On the other hand, the peeling reaction does not effect acid and neutral bisulfite pulping because liquor directed toward dissolving lignin, attacks the cellulose chain very little, but hemicelluloses are attacked to a high degree, especially when the cook is greatly prolonged in order to remove as much of the lignin as possible before bleaching.

It is therefore a general object of this invention to provide a process for increasing the yield of pulp in acid and neutral bisulfite cooks. A further object is to produce a pulp having good brightness stability while maintaining increased yield without substantially impairing other desirable properties. Another object of this invention is to provide a process for rapidly obtain ing an increase in pulp yield of acid and neutral bisulfite pulp cooks.

Other objects, features and advantages will be evident from the following disclosure.

SUMMARY OF THE INVENTION It has been found that pulp yield produced under acid and neutral bisulfite pulping conditions may be increased by treating the lignocellulosic materials, e.g.,

wood chips, with a solution of cyanide and bisulfite. The process may be accomplished through several em bodiments. The first of which is to add the cyanidebisulfite ion-containing solution to the lignocellulosic material and digest as a normal acid or neutral bisulfite cook. A second embodiment is to treat the chips with a mixture of cyanide-bisulfite ions in solution at a temperature below 130C. preferably about C., and raise the temperature to digesting conditions. A third embodiment is to treat the chips with a solution of cyanide ions and bisulfite ions at a temperature below about C. and preferably below 75C., withdraw the excess cyanide disulfite solution, add bisulfite liquor and digest under normal digesting conditions. This third embodiment is preferred because the cyanidebisulfite solution is not subjected to the time, temperature and other conditions optimum for digesting. Also, losses of solution by side reactions are thereby eliminated.

It was found the treatment of chips with a solution of cyanide-bisulfite ions results in yield increases in acid and neutral bisulfite cooks over the untreated acid and neutral bisulfite cooks. Also the reaction to form the cyanohydrin group occurs faster with the bisulfite present. Additionally, brightness stability is greater for cyanide treated unbleached pulps than for untreatedpulps.

DETAILED DESCRIPTION OF THE INVENTION This invention increases the yield of the cellulosic and hemicellulosic components of bisulfite pulp. The term yield as used herein is the dry weight of washed fibers which are recovered after pulping as a percentage of the weight of dry lignocellulosic material which was charged to the digestion process. Any increase in yield caused by loss of lignocellulosic material is gener ally undesirable in papermaking.

It has been found that if a sulfite is present with cyanide during treatment of lignocellulosic material the initially formed bisulfite adducts with the carbonyl groups, will be subsequently displaced with the cyanide ions to form cyanohydrin groups. This results in a faster formation of the cyanohydrin group In the case of reaction with carbonyl groups in lignin, the result is improved dissolution rate of lignin. The process of this invention therefore comprises the steps of treating lignocellulosic material with a solution of cyanide ions and bisulfite ions to form cyanohydrin groups byreaction of the bisulfite ion with aldehyde end groups of the lignocellulosic material and the cyanide ion with the bisulfite adduct to form a cyanohydrin group. To maintain satisfactory contact between the cyanide bisulfite solution and the lignocellulosic material, a solution to material ratio of about 4 to l is preferred but is not critical as long as sufficient contact is made. The reaction of bisulfite with the lignocellulose carbonyl group occurs rapidly to form the bisulfite adduct. The reaction of cyanide alone with a carbonyl occurs slower than when the cyanide displaces the bisulfite adduct as shown below and described in Organic Chemistry, Morrison and Boyd, 2nd Edition, (Allyn and Bacon, Inc., Boston 1967) at page 638:

The treatment of the lignocellulosic material may" be accomplished by several methods. The first is to treat the lignocellulosic material with the cyanide-bisulfite solution as a pulping liquor and heating at about 150-180C. for from 2 to 4 hours. Although the cyanohydrin group is formed more rapidly following this embodiment, hydrolysis of the cyanide ion into ammonia and formic acid is also accelerated at the higher temperature. This undesirable reaction is somewhat offset by the presence of the bisulfite reaction.

Another embodiment of the process of this invention is to contact lignocellulosic material with a cyanidebisulfite ion solution at a temperature below 130C., preferably 95C. and follow-up by raising the temperature to conventional bisulfite digesting temperature.

A third, and preferred, embodiment of this process is to react lignocellulosic material with a cyanide-bisulfite ion solution at a temperature below 130C., preferably below 75C., until the cyanohydrin group is formed, withdraw the excess cyanide-bisulfite solution, add bisulfite pulping liquor, and digest as usual.

As a practical digestion procedure for wood chips, diffusion of cyanide-bisulfite solution throughout the cellulosic material and eyanohydrin formation generally proceed simultaneously. lf wood chips are presteamed to drive out air and thoroughly wet the wood structure, about 70 percent of the possible reaction occurs in 3 hours at a temperature of 95C. At a temperature of 130C., approximately minutes is sufficient, but'control and uniformity of cooking conditions are not reliable. An economic penalty must also be paid for cyanide ion which becomes unavailable for cyanohydrin formation because of hydrolysis directly into ammonia. The aqueous cyanide ion containing solution should have at least 2.0 grams of bisulfite per liter, preferably sodium bisulfite for the most favorable reaction, to keep the reaction at an acid pH. To achieve the most favorable conditions, the cyanide ions should be contacted at a concentration of 0.3 grams of cyanide ions per liter of the solution. Any aqueous solution containing cyanide ions may be used. When KCN, for example, is added to water, it readily dissociates into K ion and 5 CN' ion. Whether derived from l-lCN gas, liquid hydroi gen cyanide, an inorganic compound or an organic compound, any treatment solution therefore contains reactive CN ions, so the source of the cyanide ion is j immaterial. The more CN ions to bisulfite ions in the 1 treating solution the higher the pH. There should not;

be such a disproportionate amount of CN ion that the pH is above 7.0.

This invention may be practiced in continuous or. batch digester operations. One embodiment for carrying out the invention, which is suitable for batch or continuous operations, is: (a) presteam chips to drive out air, soften wood, and fill pores with liquid; (b) in- 3 troduce a solution of sodium cyanide-sodium bisulfite at a convenient concentration for a treatment which varies inversely in duration with the temperatures; (c) 3 this invention, in addition to increased yield, is the brightness stability of the resulting pulp.

The practice of this invention may clearly be seen in the following examples.

EXAMPLE 1 This example illustrates a comparison of normal acid bisulfite cooks to cyanide-bisulfite pretreated, acid bisulfite cooks. The cooks were made using 200 gram (O.D.) samples ofloblolly pine. The chips for each normal acid bisulfite cook (cooks l-3) were placed in one compartment of a multiple unit laboratory rotating digester and sodium bisulfite cooking liquor was added at a 4:1 liquor to chip ratio. The cooking conditions are shown in the table below. After each cook was completed, the digester was cooled. The chips were fiberized and dried. Yield, Kappa No. and total lignin determinations were made. The results are shown in the table below.

For comparison Cooks 4 and 5 were made by pretreating 200 gram samples of loblolly pine chips with a solution of sodium cyanide, 1 1.3 grams, and sodium bisulfite, 67 grams at 100C. for 85 minutes and 60 minutes respectively. The pretreating solution was drained off and 67 grams of sodium bisulfite in 37.3 ml. of water was added and the chips cooked for the prescribed time. After the cook was completed, the digester was cooled. The chips were filtered and the pH taken of the liquor. The chips were placed in a Waring 30? Cook Number 1 2 3 4 5 Liquorto chip ratio..... 4:1 4:1 4:1 4:1 4:1 Cyanide consumed percent on CD. wood 3.0 3.0 Applied Nal-lSO as percent SO; on O.D. wood 15 15 15 15 15 Time to maximum temperature. min 25 20 30 Time at 170C.

minutes 120 30 120 30 pH at beginning of cook 4.0 4.1 pH at end of cook.. 3.0 3.3 3.4 5.3 5.4 Kappa number 45.1 147 137 138 Total lignin percent on O.D. pulp 13.75 18.60 22.1 21.5 22.6 Total pulp yield percent on O.D. wood 51.2 62.3 71.3 79.7 85.0 Pretreatment time,

min 60 i The results in the table show a substantial increase in yield of pulps pretreated with a solution of bisulfite and cyanide ions in the 20-23 percent total lignin content range over untreated chips.

EXAMPLE 2 This example illustrates a comparison of normal neutral (buffered) bisulfite pulp to cyanide-bisulfite pretreated cooks neutral cooks. Cooks 6-8 were made using 200 grams (O.D.) samples of loblolly pine. These cooks were made by placing the samples in a multiple unit laboratory digester with sodium bisulfite liquor at 15 percent S0 on O. D. wood and buffered with 48.5 grams of Na l-lPO, and 54.5 grams of Na PO Each sample was treated for 1 hour at C. The temperature was then raised to 170C and the cooking time was of the pulps was very different. The normal acid bisulvaried. After each cook, the digester was cooled, the fite pulps had the familiar reddish appearance normally chips fiberized, washed and dried. Yield, Kappa No. associated with sulfite pulps. Neutral-bisulfite pulps and lignin content of each sample was determined and treated with cyanide-bisulfite were very pale yellow the results shown in the table below. 5 colored immediately following the cook, but quickly For comparision cooks 9 and were made by prereverted to a medium brown color.

treating the pine chips with a cyanide-bisulfite solution While the invention has been described and illusof Example 1 for one hour at 100C. and raising the trated herein by references to various specific matericooking temperature to 170C. for the prescribed als, procedures and examples, it is understood that the cooking time. These cooks were treated the same as the 10 invention is not restricted to the particular materials,

untreated cooks and the results are shown in the table. combinations of materials, and procedures selected for that purpose. Numerous variations of such details can Cook Number 6 7 g 9 10 be employed, as will be appreciated by those skilled in the art. ll;];EetOcgglSl:"ll:$:1O.n-. 4:1 4:1 4:1 4:1 4:1 What is claimed is:

percent on 0D. 1. A process for increasing the pulp yield of lignocellulosic material which comprises treating the lignocelpercent 302 on lulosic material with a solution containing cyanide ions 4 15 15 15 15 15 and bisulfite ions at a pH below 7.0 and a temperature i inhzi t iii e l m i n 25 0 35 30 3O between about 75C and 130C., withdrawing the ex- Time at 170C, min..... 90 120 30 120 30 cess solution, and digesting the thus treated lignocellui of 6 9 6 9 6 7 6 7 losic material with a bisulfite pulping solution. pH at e rid o f cobk: llll n 6.8 6:8 6:8 7:0 6:8 2. The process of claim 1 wherein the lignocellulosic xappafiulnber 151 146 143 material is in the form of wood chips. Total llgnin percent on on pulp 2155 199 225 185 20A 3. The process of claim 1 wherein said treating IS con- Total pulp yield ducted at a temperature below about 95C.

82] 77] 84. BM 4. The process of claim 1 wherein said cyanide ion is lretreatment-time, obtained from a member of the group consisting of po- 80 75 75 75 75 tassium cyanide, sodium cyanide and hydrogen cyanide. The results show substantial increase in yield of cya- 5. The process of claim 1 wherein said cyanide ion is nide-bisulfite pretreated neutral pulps over normal at a concentration of at least 0.3 grams per liter in said neutral (buffered) pulps. solution and said bisulfite ion is at a concentration of j at least 2.0 grams of sodium bisulfite per liter of solu- EXAMPLE 3 .35 tion, said solution to said lignocellulosic material being The G. E. brightness was measured on pulp samples at least 4 to 1 by weight.

. 3 and 7-10 of Examples 1 and 2. The brightness reten-. 6. A process for increasing the pulp yield lignotion of the pine pulps was determined after heating for cellulosic, material which comprises, cooking said lig- 1 hour and 2 hours at 105C. nocellulosic material for from 2 and 4 hours at a tem- Cook No. 3 7 8 9 ll) Normal Cyunide-Bisulfite acid Normal beutral pretreated neutral Type bisulfite bisulfite bisulfite G. E. Brightness: Initial 35.8 36.4 38.8 26.8 27.3 After] hr. at 105C 34.0 34.7 35.9 25.4 25.8 After 2 hr. at 105C 32.1 32.3 33.1 25.3 25.5

The initial brightness of cyanid e bisulfite pretreated 50 perature between about 75C and 180C. in a solution neutral bisulfite pine pulps was higher than untreated, of cyanide ions and bisulfite ions at a pH below 7.0. convention neutral bisulfite pulp. Also, the appearance Patent No. 3,762,990

- I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated October 2, 1973 i Sten 1. Fa'lkehaq, A'lan M. Bins, HaroId L. Hintz It is certified thaterror appears in the above-identified patent and} that said Letters Patent are hereby corrected as shown below:

Co'lufnn 2, N ne 65, the chemica] reaction should read o 0H I an E I R- C HSU R C H50 CN' R C CN I v "-1 RI. I a I R' I R' carbonyl bi sulfite v cyanohydrin I I adduct I C ohjmn 5, HI the tabIe appearing with examp1e 3, "35. 8" sheuId read 36.8 l Q sighed and sealed this 16th day of ul 1971,.

(SEAL) Attest:

McCOY M. GIBSON, JR. C. MARSHALL DANN I Attesting Officer I Commissioner of Patents 

2. The process of claim 1 wherein the lignocellulosic material is in the form of wood chips.
 3. The process of claim 1 wherein said treating is conducted at a temperature below about 95*C.
 4. The process of claim 1 wherein said cyanide ion is obtained from a member of the group consisting of potassium cyanide, sodium cyanide and hydrogen cyanide.
 5. The process of claim 1 wherein said cyanide ion is at a concentration of at least 0.3 grams per liter in said solution and said bisulfite ion is at a concentration of at least 2.0 grams of sodium bisulfite per liter of solution, said solution to said lignocellulosic material being at least 4 to 1 by weight.
 6. A process for increasing the pulp yield ligno-cellulosic, material which comprises, cooking said lignocellulosic material for from 2 and 4 hours at a temperature between about 75*C and 180*C. in a solution of cyanide ions and bisulfite ions at a pH below 7.0. 